The goalposts coming down in '88 against the Jets. You just knew it was the start of something special.
Second favorite was the '02 Miami snow game. It was my wife's (gf at the time) first Bills game. She was a born and raised San Diegan and had never even spent time in the snow. It was the quintessential Buffalo Bills experience (tailgate, booze, snow, comeback against divisional rival, Kelly whooping it up on the sideline, etc.) and she absolutely loved it!